For each paper making process a correlation exists between fiber coarseness and product quality in terms of product softness or handfeel. Expensive high quality fibers such as bleached northern kraft softwood fibers are fine, flexible, and produce high quality tissue products. In contrast, mechanical pulping of softwoods produces coarse, stiff fibers typically used in making newsprint. Newspapers contain a preponderance of coarse, high yield fibers, typically stone groundwood (SGW), thermomechanical pulp (TMP), and/or chemithermomechanical pulp (CTMP) fibers. Such coarse newsprint fibers are usually highly refined to cause fractures and fibrillations which aid in imparting strength to the resulting newsprint. Such refining changes the "freeness" of the coarse fibers from "high" freeness fibers to "low" freeness fibers. If such refined, coarse mechanical fibers are used in a tissue making process the resulting sheet is not soft, and therefore has poor tissue properties. A recent thorough discussion of the relationship between tissue softness and fiber coarseness is contained in Canadian Patent No. 2,076,615.
Attempts to produce soft tissue or towel type sanitary paper products from a majority of high yield fibers such as CTMP, TMP or SGW pulp have not been successful. Likewise, producing soft tissue and towel products by recycling old newspapers has not been very successful partially because the predominant fiber in newsprint or in old newspapers are low freeness, coarse, high yield fibers.
Other complicating factors in producing soft tissue and towel products by recycling old newspapers are difficulties with the paper machine operation caused by poor drainage associated with low freeness fibers, and problems caused by high amounts of fines and other substances, which separate from the fibers and accumulate in the paper machine water system (white water). These materials make it difficult to crepe the tissue sheet from the Yankee drying cylinder, and therefore necessitate operating the paper machine at conditions which do not promote maximum softness.
The present invention solves these difficulties by enzymatically modifying the fibers and by adding oils of the type used in newspaper ink to the high coarseness fibers, thereby softening the fibers and giving them release properties which aid in the creping step on the paper machine. For recycled newsprint, the fibers are enzymatically modified, and oils of the types used in newsprint ink are added, and a portion of the printing oils in the fibers are retained, thereby softening the fibers, and making the fibers more flexible which aids in the creping step on the paper machine. Consequently, previously unachievable levels of tissue and towel softness are possible with the present invention using high coarseness fibers or recycled newsprint fibers.
Methods to improve softness of tissue products using low coarseness fibers by lotionizing the surface of the tissue product have been developed. These methods, however, are applied to the tissue product prior to it being converted into the final product. It would be desirable to provide a method of achieving these softness gains without the cost and complexity of the current methods.
Conventional recycling of old newspapers to obtain fibers comparable to the type of fibers used to originally make the newsprint is known in the art as "deinking" and typically involves pulping, washing (usually with surfactants), screening, centrifugal cleaning, solubilizing insoluble contaminants (usually by strong caustic treatments), washing and bleaching of the fibers to counteract the yellowing effects of caustic treatments.
The first step in conventional recycling of old newspapers is to separate the paper into individual fibers in water to form a pulp slurry. Surfactants and caustic are added to facilitate the solubilization and separation of contaminants from the fibers. This is followed by removing inks and contaminants from the fibers by a combination of various process steps such as screening, centrifugal cleaning, washing, flotation and the like. The screening and centrifugal cleaning steps remove large contaminants such as paper clips, staples, plastics, etc. The primary purpose of washing and flotation steps is to suspend contaminants in the water and to remove the contaminants from the fibers. When caustic is used to facilitate contaminant removal, some yellowing of the fibers unfortunately occurs due to the caustic treatment. After or during caustic treatment and washing, the fibers are usually bleached (e.g.--with hydrogen peroxide) to counteract the yellowing effect of caustic or to produce better fibers having higher brightness than the fibers in the original waste paper. Cleaned, decontaminated, and bleached fibers are usually blended with virgin fibers and then used in a paper making process for which the fibers properties are suitable. Because the starting fibers are newsprint type fibers, i.e., coarse, low freeness and low brightness fibers, such recycled fibers are most often reused for making blank newsprint. They are generally not suitable because of their high coarseness and low freeness for making soft tissue products unless blended with a majority of higher quality fibers such as bleached northern softwood kraft pulp.
Conventional pulping of used newspaper to obtain recycled newsprint fiber is usually done in a high attrition pulper at a consistency of 4-8% and at 90.degree. F.-160.degree. F. for 20 minutes to 60 minutes, depending on the exact type of waste paper being processed. Caustic soda or other alkaline substances such as sodium silicate are commonly used to raise the pH of the pulp slurry to pH 9-10 to aid in separating fibers (defibering) and also to loosen the inks and separate dirt from the fiber. At an alkaline pH vegetable oils in the inks are saponified while mineral oils are emulsified by the combination of alkaline pH, soaps, and surfactants, all of which enhance the removal of oils during washing. A surfactant deinking aid (for higher pH ranges) is usually added to further help separate inks from fiber.
The caustic step in recycling processes of old newsprint to obtain well cleaned quality fibers causes swelling of the fibers, and usually solubilizes many components. In addition to saponifying vegetable based printing oils, caustic also saponifies natural organic acids typically present in old newspapers to produce the corresponding soaps of the saponifiable materials. The saponified vegetable oils and organic acids so formed aid in removal of other contaminants from the fibers, such as non-saponifiable printing oils (mineral oil). These substances are subsequently removed from the fibers by washing and/or flotation after the caustic treatment.
A major recycler of old newspapers, Garden State Paper, in recent journal articles, one entitled "The Big "D": Getting Rid of the Ink in Recycled Fiber appearing in the journal Paper Age, 1991 Recycling Annual, at pages 23 and 50 and the other article entitled "Recycling From the Newsprint Perspective, at pages 9, 12 and 13 of the same 1991 Recycling Annual, (Paper Age, 1991 Recycling Annual) describes its newsprint recycling and deinking process as cleaning and screening followed by a series of 3 washings facilitated by the addition of chemicals to emulsify the printing oils and resins. Again the aim of this process is to remove printing ink constituents including oils as completely as possible. This is especially important because the recycled newsprint fiber is made into blank newsprint paper which would not have adequate brightness or strength without removing the ink constituents.
A common component of deinking systems for newspaper waste involves separating ink from the fibers and removing the ink typically through washing and flotation steps. While conventional alkaline deinking chemicals are very effective in such deinking, they are known to have the disadvantage of lowering brightness. Recent research has been directed to avoiding alkaline deinking chemicals in deinking systems.
Recent developments in wastepaper deinking (U.K. Patent Application 2,231,595 published Nov. 21, 1990 entitled "Deinking Water Printed Paper Using Enzymes" and a North Carolina State University publication entitled "Enzymatic Deinking of Flexographic Printed Newsprint: Black and Colored Inks") deal with the use of enzymes to aid in the detachment and removal of inks from the fibers. These processes describe the use of enzymes such as cellulase, pectinase, xylanase, and hemicellulases to facilitate ink removal without the negative effects of caustic treatment on brightness along with the use of flotation to remove the agglomerated ink particles. Since printing oils are lighter than water, they are readily removed by flotation treatment particularly in view of the chemicals added to aid in separation. While enzymes are used, this thorough removal of ink components is counter to the objective of the present invention which retains the oils for tissue softness. A paper presented at the Fifth International Conference on Biotechnology from May 27 to May 30, 1992 in Kyoto Japan entitled "Enzyme Deinking of Newsprint Waste" by John A. Heirmann, Thomas W. Joyce and D. Y. Prasad described research occurring at the department of Wood and Paper Science, North Carolina State University, Raleigh, N.C. That article described the use of acidic flotation deinking systems in which the only chemicals used were enzymes, calcium chloride and a surfactant. The enzymes were a preparation containing both cellulase and hemicellulose. Increases in freeness and brightness were noted. However, the important distinction is that the acidic flotation deinking system described removes ink along with its associated oils which is contrary to the present invention.
More recently, high consistency pulping (13-18%) has been utilized for recycling old newspapers. This type of pulping technology utilizes the additional effect of rubbing/kneading between the fibers/papers at higher consistency to defiber and assist in separating inks from the fibers. Generally the pulping temperature, time, and chemical additions are the same as lower consistency pulping described above.
One aspect of the present invention avoids conventional deinking but instead retains a significant component of the ink, i.e., the printing ink oils. The present invention is based on the discovery that if the oily component of ink is not removed from coarse fibers in old newsprint, surprisingly high quality, soft tissue products can be produced. To accomplish this task, a formulation of enzymes is utilized to loosen a limited amount of the ink constituents for removal and/or redistribution on the fibers. In addition, by avoiding saponification conditions, e.g. alkaline saponification of fatty acid oils such as vegetable oils, and fiber components such as hemicellulose are not allowed to leach out of the fibers into the paper machine water system and cause difficulties with the creping operation.